linux-kernel - Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp

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Date: Thu, 9 May 2024 17:13:12 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, RCU <rcu@...r.kernel.org>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

On 05/07, Paul E. McKenney wrote:
>
> On Tue, May 07, 2024 at 10:54:41AM -0400, Oleg Nesterov wrote:
> > Hello,
> >
> > I feel I don't really like this patch but I am travelling without my working
> > laptop, can't read the source code ;) Quite possibly I am wrong, I'll return
> > to this when I get back on May 10.
>
> By the stricter data-race rules used in RCU code [1], this is a bug that
> needs to be fixed.

Now that I can read the code... Sorry, still can't understand.

> which is read locklessly,

Where???

OK, OK, we have

	// rcu_sync_exit()
	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0)

and

	// rcu_sync_dtor()
	WARN_ON_ONCE(READ_ONCE(rsp->gp_count));

other than that ->gp_count is always accessed under ->rss_lock.

And yes, at least WARN_ON_ONCE() in rcu_sync_exit() can obviously race with
rcu_sync_enter/exit which update gp_count. I think this is fine correctness-wise.

But OK, we need to please KCSAN (or is there another problem I missed ???)

We can move these WARN_ON()'s into the ->rss_lock protected section.

Or perhaps we can use data_race(rsp->gp_count) ? To be honest I thought
that READ_ONCE() should be enough...

Or we can simply kill these WARN_ON_ONCE()'s.

I don't understand why should we add more WRITE_ONCE()'s into the critical
section protected by ->rss_lock.

Help! ;)

Oleg.


which in turn results in a data race.  The fix is to mark
> the updates (as below) with WRITE_ONCE().
>
> Or is there something in one or the other of these updates to ->gp_count
> that excludes lockless readers?  (I am not seeing it, but you know this
> code way better than I do!)
>
> 							Thanx, Paul
>
> [1] https://docs.google.com/document/d/1FwZaXSg3A55ivVoWffA9iMuhJ3_Gmj_E494dLYjjyLQ/edit?usp=sharing
>
> > Oleg.
> >
> > On 05/07, Uladzislau Rezki (Sony) wrote:
> > >
> > > From: "Paul E. McKenney" <paulmck@...nel.org>
> > >
> > > The rcu_sync structure's ->gp_count field is updated under the protection
> > > of ->rss_lock, but read locklessly, and KCSAN noted the data race.
> > > This commit therefore uses WRITE_ONCE() to do this update to clearly
> > > document its racy nature.
> > >
> > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > Cc: Oleg Nesterov <oleg@...hat.com>
> > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > > ---
> > >  kernel/rcu/sync.c | 8 ++++++--
> > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> > > index 86df878a2fee..6c2bd9001adc 100644
> > > --- a/kernel/rcu/sync.c
> > > +++ b/kernel/rcu/sync.c
> > > @@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
> > >  		 * we are called at early boot time but this shouldn't happen.
> > >  		 */
> > >  	}
> > > -	rsp->gp_count++;
> > > +	WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
> > >  	spin_unlock_irq(&rsp->rss_lock);
> > >
> > >  	if (gp_state == GP_IDLE) {
> > > @@ -151,11 +151,15 @@ void rcu_sync_enter(struct rcu_sync *rsp)
> > >   */
> > >  void rcu_sync_exit(struct rcu_sync *rsp)
> > >  {
> > > +	int gpc;
> > > +
> > >  	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
> > >  	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
> > >
> > >  	spin_lock_irq(&rsp->rss_lock);
> > > -	if (!--rsp->gp_count) {
> > > +	gpc = rsp->gp_count - 1;
> > > +	WRITE_ONCE(rsp->gp_count, gpc);
> > > +	if (!gpc) {
> > >  		if (rsp->gp_state == GP_PASSED) {
> > >  			WRITE_ONCE(rsp->gp_state, GP_EXIT);
> > >  			rcu_sync_call(rsp);
> > > --
> > > 2.39.2
> > >
> >
>